Sulfide method plutonium separation



SULFIDE METHOD PLUTON-IUIVI ISEBARATION Robert B. Dumeld, Champaign,111., assignor to-tlt'e United States of America as represented by-theUnited'States Atomic Energy Commission No Drawing. Application May4,'194'8 Serial No. 25,075

6 Claims. c1. 23-1455 This invention is concernedwith.a.method..for.separat-- ing plutonium from impurities normallyassociated:there-- with in neutron-irradiated uranium.

In this specification and claims .thename of'theelement is used todesignate the element generically, eitherxin its element state orcombined in ateompoundunless other wise indicated by the sense in whichit is used, for. example by a specific designation, such as -metal?or.elemental.

Natural uranium is composed of. three. isotopesnamely, U U and U3 the.latter being present in! excess of 99% of the whole. When. Uis.subjected-to the action of slow or thermal neutrons.aiourthLisotope,U2 is produced havingahalf-life.ofttwentyethree. minutes which undergoesbeta decay. to Np which in..turn

decays further by beta radiation .with. ahalflifeofjtwor andthree-tenths days to yield. plutonium... In addition. to the formationofPu there arezsimultaneously produced vother. elements of loweratomicweight .knownasfission they quickly transform themselves intoisotopes-of these various elements having longer half-lives.Themesulting' materials are commonly knownasfission products... Thevarious radioactive. fission .products have half-livesranging from. afraction of a second to. thousands ofiyears. Those having half-liveswhichare .very shortunaybesubstantially eliminated by aging. thematerial for areasonable period before handling. Those-withverylonghalflives do not have sufficiently intensenradiationsto endangerpersonnel protected by moderateshie'ldingn on the other hand, thefissionv products. having haltllives ranging from a few days to afew'years havedangerously intense radiations which cannot.beelirninatedi'bypaging for practical storage. periods. These productsare-chiefiy radioactive isotopes of Te,-.I, Cs, Ba,.La,-Ce,.Sr',v Zi",Cb, and Rh.

It may be readily seen that plutonium, producedas" generally setforth-above, is contaminated with,.considerable quantities of uraniumand fission products. In fact, the plutonium constitutesonlyxa'veryxminorfportion of the irradiated mass, i. e., less.than-.=1%thereof. .Intview 6Q of such a low concentration ofplutonium;imtheiirradiatedi metal it becomes apparent that the procedureemployed to recover that element must be highly efficient in order to beat all practicable. In addition to the recovery of the plutonium, it ishighly desirable to separate the fission products from the plutonium,since these have exceedingly important uses as therapeutic agents andare also used as radioactive tracers in chemical, medical, andindustrial research.

It is an object of this invention to provide a method of separatingplutonium from an aqueous solution in which the plutonium is containedin a small concentration.

An additional object of this invention is to provide' a convenientandefiicient method of recovering plutonium from impurities commonlyassociated therewith in neu-' tron-irradiated uranium. V

Broadly, the present invention"involve'sthe treatment of'plutonium in alower state of oxidation; -with "a soluble'sulfide in the presenceofa'source-of uranylions. The plutonium which is contained in anaqueous, alkaline solution, is contacted with a soluble sulfidecompound,whereby a precipitate is formed whicheontains-a substanti'al'part of theplutonium present. The-precipitate is separated from the solution, thuseffectively separatingthe plutonium from such impurities as are solublein thelsolution under these conditions; The preeipitateisithenrdissolved by treatment withan'aqueous soluti'omco'ntainingcarbonate ion in eXcess of that stoichiometrically .re quired to form asoluble carbonate :complex withi-the uranium and the plutonium containedin the precipitate. This treatment brings the plutonium and uranium intosolution, but does not affect the fission product sulfides which are notsoluble in anexcess carbonate solution; for example, the eerie sub-grouprare earth" sulfides; The solution is then centrifuged, thus separatingth'eiplutonium from these insoluble fission product sulfides;

In its preferred embodiment, this'pro'cess' comprises the treatment ofplutonium, contained inan aqueousalk'aline solution, in a reducedstate,,i. e.', the'tri-i ortetravalent state. This solution maybeformedby; dissolving neutron-irradiated uranium in nitric "acidand'ithen'converti'ng the solution to the alkaline state .by' additionof "analkali hydroxide. The solution formedin this "manner will'containthe plutonium in very'small' concentration, uranyl ions and fissionproducts: Theplutonium-containing solution isthen treated with a solublesulfide, and preferably' hydrogen sulfide, whereby; aninsoluble"precipitate is formed." This precipitate contains' theuranium;plutonium, andthose fission products "present; which'ifo'rminsolublesulfidesin alkaline solution. The'fissionproducts,

such .as S1", Te, Rb, and Cs, whichi'form" soluble" sulfides inalkalinesolution, will remain in'solution andhythe separation of the insolubleplutonium-eontainingprecipitate from the solution, the plutonium'willb"e"efi'ective1y freed from this type of fission products; Separationof'the precipitate may be effected by any'of 'the"usual'methods,

45 such: as 'centrifugation, decantation; or'filtration. Following theseparation of the precipitate; it may be washed with water and thentreated with a soluti'dn'containing carbonate ion in excess. Theplutonium in its lower oxidation-statesforms a very stablecomplexwith'carbonate 0ion and .the carbonate solution thus willeffectively dissolve the. plutonium. Uranyl ion is dissolvedinth'e sameway. and so is also brought into solution; Certain'of the fissionproducts which form insoluble sul'fi'des, notably cerium, lanthanum, andthe other members" of theceric 5 sub-group of the rare earth, areinsoluble' nexcesscarbonate, so these willremain in the solid form asthe sulfides, in the presence .of the excess carbonate-solution. Theseundissolved sulfides may be separated 'frorn thies'olution by,eentrifugation, filtration, or'decantation': This step, willelfectively'separate" the plutonium from the greater portion oftheremaining"contaminants: Plutonium may then be separated from theuranyl ion-containing solution by any of the usual methods, such asprecipitation with a lanthanum fluoride or bismuth phosphate carrier.The process of this invention has been described as applied to theseparation of plutonium contained in solution in such smallconcentration that it must be precipitated with an auxiliaryprecipitate. The process may be directly precipitated. In case theplutonium is present in such concentration the procedure for separatingthe plutonium is the same as that previously set forth, except that theprecipitation of the plutonium may be carried out in a solution whichdoes not contain uranyl ions.

The advantages of the process of this invention are readily apparent. Itprovides a simple method for separating plutonium from solutions inwhich the plutonium is contained in small concentrations. It alsoprovides a novel method of separating plutonium from such contaminatingelements, normally associated with plutonium in neutron-irradiateduranium, as form soluble sulfides in an aqueous alkaline solution. Itfurther provides a method of separating plutonium from the contaminatingelements, the sulfides of which are insoluble in an excess carbonatesolution. The separation method is a simple one, which does not requirethe use of any highly corrosive reagents and efl'iciently separatesplutonium from some of the most highly radioactive fission products.

Now that this invention has been broadly described, it may be furtherillustrated by the following specific examples.

Example I A 0.5 M potassium hydroxide solution was prepared in a volumeof about cc. To this solution was added 0.5 mg. of potassium diuranateobtained from neutronirradiated uranium, containing a tracer quantity of50- year plutonium in the tetravalent state. Hydrogen sulfide was thenpassed into the solution for ten minutes. The uranyl precipitate thusformed was then separated from the solution by centrifugation. Theprecipitate and the supernatant solution were analyzed and it was foundthe plutonium had carried with the precipitate to the extent of 81% with12% of the plutonium unaccounted for and the balance remaining in thesupernatant solution.

Example 11 A 0.5 mg. sample of the uranyl precipitate obtained in themanner described in the preceding example was introduced into 5 cc. of a0.5 M potassium hydroxide solution. This solution was then made 0.5 M inK CO and 0.6 M in NH OH. The precipitate was digested for about thirtyminutes in this solution, which treatment efiected the dissolution ofthe greater part of the precipitate. The balance of the undissolvedprecipitate was then separated from the solution by centrifugation andthe precipitate and solution analyzed by radiometric methods. Thesolution was found to contain 91% of the plutonium contained in theoriginal precipitate with 6% remaining in the undissolved precipitateand 3% unaccounted for.

It will be apparent to those skilled in the art that variousmodifications in the present invention exist. In general, it may be saidthat any process for the isolation of plutonium, wherein the plutoniumis separated from an alkaline solution either directly or together witha uranyl sulfide carrier, is to be considered as lying within the scopeof the present invention.

What is claimed is:

1. The process of separating plutonium from contaminants normallyassociated therewith in neutron-irradiated uranium, which comprisestreating an aqueous alkaline solution containing plutonium values andcontaminants normally associated therewith, said plutonium having aValence state less than +5, in the presence of a source of 'uranyl ions,with a soluble sulfide, whereby a plutoniumcontaining uranyl precipitateis formed, separating said precipitate from the solution, treating saidprecipitate with an aqueous solution containing carbonate ion in excess,whereby the plutonium is dissolved away from the fission productsulfides insoluble in excess carbonate, and separating said insolublefission product sulfides from the plutonium-containing solution.

2. The process of claim 1 in which the soluble sulfide is hydrogensulfide.

3. The process of separating plutonium values from contaminants normallyassociated with plutonium in neutron-irradiated uranium, which comprisestreating an aqueous alkaline solution containing said plutonium valuesin a valence state less than +5, fission product values and a source ofuranyl ions with a soluble sulfide, whereby a uranyl precipitatecontaining said plutonium values and insoluble fission product values isformed, separating said precipitate from the solution which containsfission product sulfides which are soluble in alkaline solution,treating said precipitate with an aqueous solution containing hydroxideions and carbonate ions in excess of the amount stoichiometricallyrequired to form a soluble carbonate complex with the uranyl andplutonium ions in said precipitate whereby the plutonium is dissolvedaway from the fission product sulfides insoluble in excess carbonate,and separating said insoluble fission product sulfides from the solutioncontaining said plutonium values and said uranium values dissolvedtherein.

4. The process of separating plutonium from contaminants normallyassociated with plutonium in neutronirradiated uranium, which comprisestreating said plutonium contained in an aqueous alkaline solution in thetetravalent state, with a soluble sulfide whereby a plutonium-containingprecipitate is formed, separating said precipitate from the solution,treating said precipitate with an aqueous solution containing carbonateion in excess, whereby the plutonium is dissolved away from the fissionproduct sulfides insoluble in excess carbonate, and separating saidinsoluble fission product sulfides from the plutonium-containingsolution.

5. The process of separating plutonium from an aqueous alkaline solutionin which plutonium is contained in very dilute concentration, in avalence state less than +5, which comprises treating said plutonium witha soluble sulfide in the presence of a source of uranyl ions, whereby aplutonium-containing uranyl precipitate is formed, and separating saidplutonium-containing uranyl precipitate from the solution.

6. The process of separating plutonium from a contaminant whose sulfideis soluble in an aqueous alkaline solution and which is normallyassociated with plutonium in neutron-irradiated uranium, which comprisestreating said plutonium contained in an aqueous alkaline solution in avalence state less than +5, in the presence of a source of uranyl ions,with a soluble sulfide whereby a plutoniumcontaining uranyl precipitateis formed, and separating said precipitate from the solution whichcontains said contaminant.

References Cited in the file of this patent Seaborg et al.: Journal ofthe American Chemical Society, vol. 70, pages 1128-1134 1948). Reportsubmitted March 21, 1942.

1. THE PROCESS OF SEPARATING PLUTONIUM FROM CONTAMINANTS NORMALLYASSOCIATED THEREWITH IN NEUTRON-IRRADIATED URANIUM, WHICH COMPRISESTREATING AN AQUEOUS ALKALINE SOLUTION CONTAINING PLUTONIUM VALUES ANDCONTAMINANTS NORMALLY ASSOCIATED THEREWITH, SAID PLUTONIUM HAVING AVALENCE STATE LESS THAN +5, IN THE PRESENCE OF A SOURCE OF URANYL IONS,WITH A SOLUBLE SULFIDE, WHEREBY A PLUTONIUMCONTAINING URANYL PRECIPITATEIS FORMED, SEPARATING SAID PRECIPITATE FROM THE SOLUTION, TREATING SAIDPRECIPITATE WITH AN AQUEOUS SOLUTION CONTAINING CARBONATE ION IN EXCESS,WHEREBY THE PLUTONIUM IS DISSOLVED AWAY FROM THE FISSION PRODUCTSULFIDES INSOLUBLE IN EXCESS CARBONATE, AND SEPARATING SAID INSOLUBLEFISSION PRODUCT SULFIDES FROM THE PLUTONIUM-CONTAINING SOLUTION.